In the future, as the number of users of wireless radio communication, such as cellular radio systems, increases and as high-speed data transmission in said systems becomes more popular, an increase in system capacity by improving the performance of the system is vitally important. A solution to this problem is to use one or more adaptive antenna groups instead of sector antennas. In an antenna group, individual antenna elements are typically located close to one another, or approximately half a wavelength apart from one another. Typically the number of antennas in such groups is divisible by two in order to alleviate the Fourier conversion, and the number of antennas is adequate in order to provide a desired coverage area. The basic principle of the method is to employ narrow radiation beams, which are directed as straight as possible towards a desired receiver. The commonly known methods used by adaptive antenna groups can be divided into two main groups: radiation beams are directed towards a receiver or the most appropriate one is selected from several alternative beams. An appropriate beam is selected for a downlink transmission or the beam is redirected on the basis of the information received from the uplink direction. The reuse of the frequencies can be intensified and the power of the transmitters can be reduced, as the directivity of the antenna beams allows to reduce the interference caused to other users.
Antenna beams are directed in a digital system by dividing a signal in the baseband parts into I and Q branches and by multiplying the signal in each antenna element in a complex manner (phase and amplitude) by appropriate weighting coefficients and thereafter by adding together the output signals in all the antenna elements. An adaptive antenna group comprises in this case in addition to the antenna a signal processor that automatically adapts the antenna beams using a control algorithm by redirecting the antenna beams into the direction of the strongest measured signal. The directivity of the beams can also be carried out in an analogue manner by generating orthogonal radiation beams using Butler matrixes and fixed phasing circuits in which the phase increases antenna by antenna. This method simply measures which beam receives most signal energy, or the strongest signal, and selects this beam for the transmission.
Publication Katz and Ylitalo, Beamspace-time Coding method for Downlink Performance Enhancement of CDMA Micro/Picocell Base Stations, Proceedings of Finnish Signal Processing Symposium (FINSIG′99), Oulu, Finland, 1999 pages 25 to 29, which is incorporated herein by reference, presents a method in which orthogonal antenna beams are used in downlink transmission as a function of an angular spread. It is an object of the invention to reduce the interference caused to other users at the same time as the downlink operation is improved utilizing Beamspace-Time-Coded transmission. In this method a signal is sent using two beams in different time slots in such a manner that a first symbol is sent using a first beam in a first time slot and a second symbol using a second beam. Thereafter a complex conjugate of the second symbol is sent using the first beam in the second time slot and a complex conjugate of the first symbol using the second beam. Since the signal is sent using two orthogonal, or uncorrelating, channels, the chance to carry out a successful transmission increases.